1. Field of the Invention
The present invention relates to a method and composition for transporting compounds including pharmaceutical compositions across the Blood-Brain Barrier (BBB).
2. Description of Related Art
The Blood-Brain Barrier (BBB) maintains a homeostatic environment in the central nervous system (CNS). The capillaries that supply the blood to the brain have tight junctions which block passage of most molecules through the capillary endothelial membranes. While the membranes do allow passage of lipid soluble materials, such as heroin and other psychoactive drugs, water soluble materials such as glucose, proteins and amino acids do not pass through the BBB. Mediated transport mechanisms exist to transport glucose and essential amino acids across the BBB. Active transport mechanisms remove molecules which become in excess, such as potassium, from the brain. For a general review see Goldstein and Betz, 1986 and Betz et al, 1994, incorporated herein in their entirety by reference.
The BBB was initially observed by Ehrlich when he observed what he termed "lower affinity" of vital dyes for the brain than other tissue. Goldmann in 1913 however, determined the actual presence of a barrier by showing that the vital dye trypan blue when injected directly into the brain stained the brain but did not leave the CNS. These early experiments by Golmann and others established that the CNS is separated from the bloodstream by blood-brain and blood-cerebrospinal fluid (CSF) barriers.
The BBB impedes the delivery of drugs to the CNS. Methods have been designed to deliver needed drugs such as direct delivery within the CNS by intrathecal delivery can be used with, for example, an Ommaya reservoir. U.S. Pat. No. 5,455,044 provides for use of a dispersion system for CNS delivery or see U.S. Pat. No. 5,558,852 for a discussion of other CNS delivery mechanisms as well as Betz et al [1994] and Goldstein and Betz [1986].
There has been some progress in designing drugs that utilize the structure and function of the BBB itself to deliver the drugs. These drugs are designed to be lipid soluble or to be "piggy-backed" into the CNS by being coupled to peptides that can cross the BBB through mediated transport mechanisms. However, not all drugs are amenable to this solution. Partridge and his colleagues have worked extensively in this area. Pharmacological formulations that cross the blood-brain barrier can be administered. [Brem et al., 1993] Such formulations can take advantage of methods now available to produce chimeric peptides in which the present invention is coupled to a brain transport vector allowing transportation of these engineered drugs across the barrier [Pardridge, et al., 1992; Pardridge, 1992; Bickel, et al., 1993]. See also The Exonomist, Jan. 4, 1997.
In the disease process, the BBB is often disrupted. For example in meningitis, Tuomanen [1993] has shown that the response against the bacterial infection lead to a breach of the BBB. Further, in trauma and brain tumors the BBB is often disrupted as well as exposure to certain agents such as soman [Lallement et al, 1991; Petrali et al, 1991]. Disruption has been shown in ischemia [Burst, 1991] and in Alzheimer's Disease [Harik and Kalaria, 1991].
In appropriate cases the blood-brain barrier disruption can be utilized to deliver drugs to the CNS, as for example osmotic disruption [Neuwelt et al., 1980a]. However, generally this is not the case since, for example, exposure to soman is accompanied by seizures [Petrali et al, 1991].
However, while these methods do provide CNS delivery for some drugs it would be useful to have additional means of delivery. In particular it would be useful to have mechanisms that temporarily and reversibly open the BBB to allow non-engineered drugs through.
Stress has been shown to affect the permeability of the BBB [Sharma, et al, 1991; Ben-Nathan, et al, 1991]. Further, in mammals, acute stress elicits a rapid, transient increase in released acetylcholine (ACh) with a corresponding phase of increased neuronal excitability [Imperato, et al, 1991]. There have been some studies showing that the pharmacological blockade of acetylcholine--hydrolyzing enzyme, acetylcholine esterase (AChE) promotes a similar enhancement in electrical activity in cortical neurons [Ennis and Shipley, 1992].
AChE has three splice variant AChEmRNAs (FIG. 1). Alternative splicing controls the generation of proteins with diverse properties from single genes through the alternate excision of intronic sequences from the nuclear precursors of the relevant mRNAs (Pre-mRNA). It is known to be cell type-, tissue- and/or developmental stage-specific and is considered as the principal mechanism controlling the site(s) and timing of expression and the properties of the resultant protein products from various genes.
Three alternative AChE-encoding mRNAs have been described in mammals (FIG. 1). The dominant brain and muscle AChE (AChE-T) is encoded by an mRNA carrying exon E1 and the invariant coding exons E2, E3, and E4 spliced to alternative exon E6. AChEmRNA bearing exons E1-4 and alternative exon E5 encodes the glycolipid phosphatidylinositol (GPI)-linked form of AChE characteristic of vertebrate erythrocytes (AChE-H). An additional readthrough mRNA (AChE-I4) species (Table 1, SEQ ID No:1) retaining the intronic sequence I4 (SEQ ID No:2; FIG. 2) located immediately 3' to exon E4 is found in rodent bone marrow and erythroleukemic cells and in various tumor cells lines of human origin. (The book Human Cholinesterases and Anticholinesterases by Soreq and Zakut (Academic Press, Inc., 1993) provides a summation of the biochemical and biological background as well as the molecular biology of human cholinesterase genes and the proteins. The book in its entirety is incorporated herein by reference.)
It would be useful to facilitate transport through the BBB by using a stress mimicking agent to have a controlled reversible disruption, or opening, of the BBB and/or blood-CSF.